Volume controlled blowing-air feed

ABSTRACT

A device for controlling a blow-molding machine for producing a thermoplastic container, wherein a blowing gas for blow-molding the container is introduced into a thermally conditioned preform, while the preform is held in a blow mold of the blow-molding machine. The blowing gas is introduced during a preblowing phase into the preform so that the temperature-conditioned material of the preform approaches the walls of the blow mold. The blowing gas is introduced during a following finish-blowing phase into the preform so that the temperature-conditioned material of the preform is pressed into the contours of the blow mold walls. The volumetric flow rate of the blowing gas is determined at least during the preblowing phase by an adjustable flow-control valve that can be controlled by a control unit that generates a control signal for adjusting the flow-control valve. The flow-control valve is adjusted as a function of the control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. patentapplication Ser. No. 14/808,686, filed Jul. 24, 2015, which claims thebenefit of priority of DE 10 2014 010 861.9, filed Jul. 25, 2014, thepriority of these applications is hereby claimed and these applicationsare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention pertains to a method for controlling ablow-molding machine for the production of a container of thermoplasticmaterial, to a control device for controlling a blow-molding machine forthe production of containers of thermoplastic material, and to ablow-molding machine for the production of containers of thermoplasticmaterial.

In methods of the type indicated above, a blowing gas or blowing air isintroduced into a thermally preconditioned preform of thermoplasticmaterial to blow-mold a container. The blow-molding process typicallyproceeds in two stages. In the first or “preblowing” phase, the preformis subjected to a first pressure of, for example, 10 bars, so that thetemperature-conditioned material of the preform expands uniformly in acontrolled manner and can come to rest against the walls of the blowmold. In the second or “finish-blowing” phase, the preexpanded preformis subjected to a second pressure of, for example, 40 bars, so that thematerial can conform completely to the contours of the blow mold beingused.

Because the pressure of the process gases at a blow-molding machine arenormally in the range of 40 bars, the blowing pressure in conventionalsystems is reduced during the preblowing phase to approximately 10 barsby a pressure control valve.

During the preblowing phase, the blowing gas is introduced into thepreform through a flow-control valve, for example, with an open crosssection of, for example, 4 mm². The setting of the flow-control valvedepends on various factors such as the type of material of the containerto be produced, the temperature profile with which the preform wasthermally conditioned, the wall thickness of the preform, and otherparameters as well. The degree to which the flow-control valve is openedis typically adjusted manually a single time and remains unchanged afterthat. Another manual change of the valve cross section can be necessary,however, when a flow-control valve has become dirty over the course ofits service life, for example, or the blow-molding machine is to be setup for a different blow mold or a different preform.

The disadvantage with this prior art is that, first, the pressure of theblowing air for the preblowing phase must be adjusted downward, and,second, the volumetric flow rate of the blowing air is not adjustableduring either the preblowing or the finish-blowing phase.

SUMMARY OF THE INVENTION

The goal of the present invention is therefore to provide a method, acontrol device, and a blow-molding machine which make it possible toproduce containers of thermoplastic material at high throughputs andwhich improve the controlled introduction of blowing air into thepreform.

According to the invention, a method for controlling a blow-moldingmachine for the production of a container of thermoplastic material isprovided, wherein a blowing gas for the blow-molding if the container isintroduced into a thermally conditioned preform while the preform isbeing held in a blow mold of the blow-molding machine. During apreblowing phase, the blowing gas is introduced into the preform in sucha way that the temperature-conditioned material of the preformapproaches the walls of the blow mold, and, in the followingfinish-blowing phase, the blowing gas is introduced into the preform insuch a way that the temperature-conditioned material of the preform isforced into the contours of the walls of the blow-mold. The volumetricflow rate of the blowing gas, at least during the preblowing phase, isset by an adjustable flow-control valve. The method according to theinvention is characterized in that a flow-control valve controlled by acontrol unit is used as the flow-control valve, a control unit generatesa control signal for setting the flow-control valve, and theflow-control valve is adjusted as a function of the control signal.

In a preferred variant, it is provided that an electrically actuatedflow-control valve is used as the flow-control valve.

When a flow-control valve controlled by a control unit is used, theintroduction of the blowing gas or blowing air into the thermallypreconditioned preform can be varied during the blow-molding phase. Theintroduction of the blowing gas can therefore be adapted individually tothe properties of the preform which determine its expansion behavior. Itshould be pointed out that the expressions “blowing gas” and “blowingair” are used synonymously. A blowing gas, in particular blowing air, isprovided for the blow-molding process.

In the invention it is advantageous that only a single blowing pressuremust be provided for both the preblowing and the finish-blowing phases.In particular, it is possible to eliminate a pressure control valve toreduce the primary blowing pressure to 10 bars, for example, for thepreblowing phase. The pressure control valves usually used to reduce thepreblowing pressure are very expensive and require continuous monitoringand/or continuous maintenance by trained personnel. The use of apressure control valve also increases the complexity of thefluid-mechanical components mounted on the blow-molding machine. Whenthe method according to the invention and the blow-molding machineaccording to the invention are used, there is no longer any need for apressure control valve.

Another essential advantage of the use of an electrically actuatedflow-control valve is that the settings of the flow-control valves forall of the blow-molding stations can be controlled from the centralcontrol system. Once the operator of the blow-molding machine hasdetermined the profile for the introduction of the blowing air into atempered preform for the production of a certain container, he cansimply enter the adjustment parameters into the central control system.There is no longer any need to set the flow-control valves individuallyfor each blow-molding station of the blow-molding machine. The controlunit preferably ensures that the flow-control valve setting, i.e., thevolumetric flow rate, is the same for each blow-molding station.

The setting of a flow-control valve by way of a central control systemeliminates the need for manual adjustment of each individualflow-control valve assigned to the blow-molding stations. A subsequentadaptation of the flow-control valve setting, e.g., for the preblowingphase, can be carried out easily and quickly by way of the centralcontrol system. This also makes it possible to adjust the flow-controlvalve for each blow-molding station individually. This can be necessarywhen, for example, a flow-control valve assigned to a specificblow-molding station has undergone aging or wear.

The control unit can be integrated into the process control of theblow-molding machine, for example. Alternatively, a separate controlsystem can be provided. The control unit is preferably integrated intothe process control of the fluid-mechanical system of the blow-moldingmachine.

To adjust the electrically actuated flow-control valve, the control unitgenerates a control signal. The electrical control signal can consist ofindividual signals separated from each other in time or a continuoussignal. Depending on the flow-control valve used or on the actuationdevice assigned to the flow-control valve, the control signal cancomprise at least some digital and/or analog electrical signals. Inparticular, it can be provided that the control signal is a controlprofile, which describes the chronological course of the flow-controlvalve settings, i.e., the change in those settings over the course oftime.

Instead of or in addition to an electrically actuated flow-controlvalve, it is also possible to use a pneumatically or hydraulicallyactuated flow-control valve. The control unit for this purpose generatesa pneumatic or hydraulic control signal as explained above, wherein thecontrol signals in this case are transmitted over pressure lines insteadof over electrical lines or in wireless fashion. In the following, anelectrically controlled flow-control valve is described as alsorepresentative of pneumatically or hydraulically controlled flow-controlvalves. The explanations and the described advantages applycorrespondingly to pneumatically or hydraulically controlledflow-control valves.

In one embodiment of the invention, the volumetric flow rate of theblowing air is controlled progressively by the use of a continuouslyvariable flow-control valve.

When a continuously variable flow-control valve is used, the advantageof being able to make fine adjustments to the volumetric flow rate ofthe blowing air is obtained. In contrast to a continuously variableflow-control valve, a stepwise-adjustable flow-control valve is low incost and of mechanically simple design. Depending on need, the methodaccording to the invention can be implemented with either astepwise-adjustable or a continuously variable flow-control valve.

The flow-control valve is preferably subjected on the inlet side to thegas pressure of the blowing air from the finish-blowing phase. In thisembodiment in particular, therefore, it is provided that a shutoff valveassigned to the finish-blowing circuit and the flow-control valve in thepreblowing circuit are both subjected to the same gas pressure. When thepressure of the blowing gas of the finish-blowing phase is used, thereis no need for a pressure reducer in the preblowing circuit of a blowingstation of the blow-molding machine. This is especially advantageous,because such pressure reducers are expensive, complex, and fault-prone.The down times of the blow-molding machine attributable to maintenancework on the pressure reducer are eliminated. It is therefore preferableto conduct the preblowing and the finish blowing with the same gaspressure, in particular without a pressure-reducing device in thepreblowing circuit. The finish-blowing circuit comprisesfluid-mechanical components for conducting blowing air for thefinish-blowing of the container, and the preblowing circuit comprisesfluid-mechanical components for conducting blowing air for thepreblowing of the container, i.e., of the preform.

In one embodiment, it is provided that a flow-control valve positionsensor detects the setting of the flow-control valve and generates aposition signal, which contains information on the position of theflow-control valve, this position signal then being transmitted to thecontrol unit.

The position information pertaining to the flow-control valve is inparticular the open cross section to which the flow-control valve hasbeen set. The transmission of the position signal to the control unitcan be conducted by the flow-control valve position sensor itself or bya transmission unit connected to it.

It can also be provided that the control unit generates the controlsignal as a function of the position signal. As a result, the controlunit can execute an automatic control algorithm, in which the controlunit determines a control deviation between a control variableinfluencing the open cross section of the flow-control valve and theactual opening of the flow-control valve and takes this deviation intoaccount in the automatic control circuit. This makes possible anautomatic control of the flow-control valve setting which is relativelyinsensitive to disturbance variables. This is especially advantageous inthe case of flow-control valves which change their flow-throughproperties as a result of aging and/or wear phenomena.

It can be provided in particular that the flow-control valve comprisesan open cross section which can be adjusted within the range from 0.8mm² to and including 20 mm².

In one embodiment of the invention, a nominal profile is stored in thecontrol unit; this profile describes the change over time in thevolumetric flow rate of the blowing air, and the control unit generatesthe control signal for setting the flow-control valve as a function ofthe nominal profile.

The nominal profile is a predetermined description of the change in theflow-control valve setting over the course of at least one time period.In particular, the nominal profile covers the time period in which theblowing air is conducted through the flow-control valve into thepreform. The change over time in the volumetric flow rate through theflow-control valve is characterized by a curve which plots theflow-control valve setting versus time. The change over time in thevolumetric flow rate of the blowing air through the flow-control valveis obtained from, for example, the pressure difference between the inletand the outlet of the flow-control valve and from the opening crosssection of the flow-control valve. The nominal profile thereforedetermines in particular the desired open cross section of theflow-control valve over time. When a flow-control valve which switchesexclusively between an open position and a closed position is used, thenominal profile preferably determines the switching frequency at whichthe valve is switched back and forth between the open and closedpositions. The nominal profile is preferably changeable.

A volumetric flow rate sensor communicating with the flow-control valvepreferably generates a volumetric flow rate signal containinginformation on the air flow quantity, wherein the volumetric flow ratesignal is transmitted to the control unit. The term “air flow quantity”is understood to mean in particular the volumetric flow rate. Theinformation on flow quantity can be further processed in the controlunit according to control and/or automatic control algorithms.

In another embodiment, a desired upper and/or lower limit value of thevolumetric flow rate of the blowing gas is stored in the control unitfor at least one point in time of the preblowing phase, wherein thecontrol unit compares the volumetric flow rate signal with the limitvalues and the generates the control signal as a function of thedifference between the volumetric flow rate signal and at least one ofthe limit values.

In this embodiment, it is provided that the upper and lower limit valuesdefine a corridor, within the boundaries of which the volumetric flowrate conducted through the flow-control valve is intended to stay. Bycomparing the volumetric flow rate signal with the limit values, it ispossible to draw a direct conclusion concerning the volume conductedthrough the flow-control valve. For the purpose of correction, thecontrol system can generate control signals to prevent overshooting orundershooting the corridor boundaries, i.e. to keep the measurementvalue of the volumetric flow rate through the flow-control valve in acertain range within the corridor. The desired limit values arepreferably changeable.

A valve with a magnetically moved closing body is preferably used as theflow-control valve. Flow-control valves of this type are, for example,RRVs (Rapid Reaction Valves). What is characteristic of these valves inparticular is that the valve seat is sealed by a spherical closing body.For the magnetic actuation of the valve, the closing body ismagnetizable, and the valve housing is surrounded by a magnetic coil.The functional principle of such valves is based on the fact that, atthe level of the closing body, i.e., for example, at the level of theball, a magnetic discontinuity is created in the valve housing. Whencurrent flows through the magnetic coil, a magnetic field acts on theclosing body at the level of the magnetic discontinuity, as a result ofwhich the closing body is lifted from the valve seat. When the currentis turned off, the flow of medium passing through the valve carries theclosing body back to the valve seat.

Valves of this type are characterized by very short switching times.These valves are preferably configured in such a way that switchingtimes of as short as one millisecond are achieved. This makes itpossible to meter the blowing air conducted through the valve in anespecially effective manner.

In particular, it can be provided that the closing body of theflow-control valve is moved back and forth at an adjustable frequencybetween an open position and a closed position. The volumetric flow ratethrough the valve can be adjusted through the choice of frequency. Inparticular, it can be provided that the valve is actuated by apulse-width-modulated signal. The flow-control valve preferably controlsthe volumetric flow rate of the blowing gas introduced into the preformduring the preblowing phase.

According to the invention, a control device for controlling ablow-molding machine for the production of a container of thermoplasticmaterial is provided in which a blowing gas for the blow-molding of thecontainer is introduced into a thermally conditioned preform, while thepreform is being held in a blow mold of the blow-molding machine. Thecontrol device comprises a control unit, which is set up to generate acontrol signal for setting a flow-control valve, which determines thevolumetric flow rate of the blowing gas. In particular, it is possiblefor the control signal to be used to set the volumetric flow rate duringthe preblowing phase.

In a preferred embodiment, the control unit is set up to generate thecontrol signal as a function of a position signal representing thecontrol position of a flow-control valve.

In another embodiment, it can be provided that, in the control unit, anominal profile is stored, which describes the change over time in thevolumetric flow rate of the blowing gas, and that the control unit isset up to generate the control signal for setting the flow-control valveas a function of the nominal profile.

A desired upper and/or lower limit value of the volumetric flow rate ofthe blowing gas is preferably stored in the control unit for at leastone point in time of the blow-molding process, and the control unit isset up to compare a volumetric flow rate signal containing informationon the flow quantity of the blowing gas introduced into the preform withthe limit values and to determine the difference between the volumetricflow rate signal and the limit values.

According to the invention, a blow-molding machine for the production ofa container of thermoplastic material is also provided, in which ablowing gas for the blow-molding of the container is introduced into athermally conditioned preform, while the preform is being held in a blowmold of the blow-molding machine, wherein the blow-molding machine isset up to implement a method according to the invention and/or theblow-molding machine comprises a control device according to theinvention.

In particular, it is possible for the blow-molding machine to compriseat least one of the fluid-mechanical components cited in conjunctionwith the embodiments of the method. This pertains in particular to theelectrically actuated flow-control valve, the flow-control valveposition sensor, and the volumetric flow rate sensor.

It has been found especially advantageous to the invention to use, asthe flow-control valve, a valve with a closing body which can be moved,especially moved magnetically, between an open position and a closedposition. It is preferably provided that the closing body is moved backand forth between the open and closed positions at an adjustablefrequency.

In one embodiment, it is provided that the flow-control valve is an RRV(Rapid Reaction Valve). In contrast to the volumetric flow rate controlby the setting of the cross section of the flow-control valve, thevolumetric flow rate in the case of valves with a closing body which canbe moved between an open and a closed position is determined by theratio between the closed and open times.

The superior features of the blow-molding machine according to theinvention and of the control device according to the invention can alsobe derived from the advantages cited with respect to the methodaccording to the invention.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of the disclosure. For a better understanding of the invention, itsoperating advantages, specific objects attained by its use, referenceshould be had to the drawings and descriptive matter in which there areillustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 shows a schematic diagram of the fluid-mechanical configurationof a blowing gas feed with continuously variable control of aflow-control valve;

FIG. 2 shows a schematic diagram of the fluid-mechanical configurationof a blowing-gas feed with continuously variable control of thevolumetric flow rate;

FIG. 3 shows a schematic diagram of the fluid-mechanical configurationof a blowing gas feed with stepwise control of a flow-control valve; and

FIG. 4 shows a schematic diagram of the fluid-mechanical configurationof a blowing-gas feed with a blowing-air control system known from theprior art in the preblowing phase.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram of the fluid-mechanical configurationof the blowing-air feed of a blow-molding machine to a blow mold 12.Inside the blow mold 12, a preform 10 is arranged, which can besubjected to blowing air through a feed line 8. The blowing air isconducted from a blowing-air supply 40 to a blowing-air distributor 42,which distributes the blowing air via the connecting ports 38 and supplylines 36 connected to them to the blowing stations, each of which has ablow mold 12. In the embodiment of the blow-molding machine as a rotarytable device, the blowing-air distributor 42 can be rotary distributor.

So that the blowing air can be conducted into the preform 10, the blowmold communicates with the blowing-air feed line 8. The blowing-air feedline 8 in the present exemplary embodiment is connected to the gas lines2, 4, and 6. The blowing-air feed line 8 can be vented via the gas line2. The valve 28 is opened to vent it. For this purpose, the controlsystem transmits a signal to the signal converter 30, which is connectedmechanically or electrically to the valve 28. When the valve 28 is open,pressure can pass from the blowing-air feed line 8 to the sound damper32 and be discharged from there to the environment.

The valves 24 and 52 are controlled analogously to the actuation of thevalve 28. Alternatively, the valves 24, 28, and 52 can be actuated bycontrol air (not shown). The signal converters 26, 30, and 54 assignedto the valves 24, 28, and 52 can be provided as status sensors, whichtransmit the open position or the closed position of the valves to thecontrol system 18.

The gas line 4 is controlled by the shutoff valve 24. The shutoff valve24 is connected to the signal converter 26. Via the gas line 4, blowingair can be conducted to the preform 10 inside the blow mold 12, this airserving in particular for the finish-blowing of the container to bemolded. In particular, the finish-blowing step takes place abruptly andat high pressure, which is applied quickly. In particular, a blowing gaspressure of 40 bars can be provided for the finish-blowing. For theabrupt introduction of the finish-blowing air into the preform 10, theshutoff valve 24 can have an open cross section of approximately 300mm². The shutoff valve 24 is preferably connected on the inlet sidedirectly, that is, without pressure reducers, flow-control valves, orthe like, to the distributor 42. Thus a blowing pressure provided by thedistributor 42 is conducted directly via the valve 24 to the blow mold12.

The gas line 6 serves to introduce blowing air into the preform 10during the preblowing phase. In the preblowing phase, the blowing air isintroduced into the preform 10 in such a way that the material of thepreform expands toward the inside walls 14 of the blow mold 12 in acontrolled manner. To set the volumetric flow rate of the blowing airduring the preblowing phase, the flow-control valve 16, configured as across-sectional control valve, is used according to the invention. Theflow-control valve 16 is connected to the control system 18 by way of asignal converter 22. The signal converter 22 receives signals from thecontrol system 18 for the setting of the flow-control valve 16. It ispreferable for the flow-control valve 16 to communicate directly with acontrol position sensor 20, wherein the control position sensor 20detects the position of the flow-control valve 16 and transmits it tothe control system 18. The signals of the control position sensor 20 canbe used to determine whether or not the flow-control valve 16 isfunctioning properly. The flow-control valve 16 preferably communicatesdirectly on the inlet side with the distributor 42. In particular, it isprovided that no pressure-reducing components such as pressure reducersor the like are arranged in the connection between the distributor 42and the flow-control valve 16.

FIG. 1 shows how preblowing is carried out during the preblowing phasewith continuously variable flow control or continuously variableautomatic flow control. It is clear on comparison with the prior art inFIG. 4 that there is no automatic pressure control valve 48 in thepreblowing circuit. Through the use of the method according to theinvention, there is no need to reduce the primary blowing pressure to 10bars, for example. The preblowing pressure is tapped directly from thefinish-blowing pressure, i.e., from the distributor 42. The distributor42 obtains the blowing gas from the gas source 40.

In the exemplary embodiment of FIG. 1, the flow-control valve 16 isconfigured as an electromechanical actuator, which is controlled by thecontrol system 18. The flow-control valve 16 transmits information viathe control position sensor 20 to the control system 18, in which theflow-control valve setting is mapped. This can be helpful, for example,when the control system 18 specifies a certain control valve value butthe flow-control valve 16 itself does not reach the desired set value.In this case, the control valve position sensor 20 reports to thecontrol system 18 that the control position deviates from the desiredvalue, and the control system 18 can then generate an error message.Independently of the concrete exemplary embodiment shown here, thecontrol system 18 can therefore be set up to generate an error messagefor displaying a defective flow-control valve 16 and/or to send itelsewhere for display.

In principle, a flow-control valve position sensor 20 is not absolutelynecessary. It is provided that, once a setting profile for theflow-control valve 16 has been stored in the control system 18, thepreblowing step will follow the same profile each time. A checkbacksignal from the sensor 20, however, is helpful with the error analysiswhen, for example, finished blow-molded containers from a certainblow-molding station have defects produced during preblowing. Theflow-control valve 16 of this blow-molding station can be singled outfor inspection. This decreases the down times of the blow-moldingmachine.

After the preblowing step, the finish-blowing of the container iscarried out. For this purpose, blowing air is conducted from thedistributor 42, through the valve 24 to the gas line 4, and finally tothe blowing-air feed line 8 and thus to the blow mold 12. During thefinish-blowing step, the important point is that, by means of an abruptincrease in pressure, the material of the preform, which has beenbrought up to the walls of the blow mold, is pressed suddenly, i.e.,very quickly and at high pressure, even more closely against the wallsof the blow mold. This ensures that the contours of the bottle will bewell formed and that the finished bottle will cool rapidly. Inprinciple, it is conceivable that the valve 24 and the associatedfinish-blowing circuit could be omitted in cases where the flow-controlvalve 16 can be opened abruptly and can thus supply the finish-blowingair abruptly into the preform. For this purpose, it would be necessaryfor the flow-control valve 16 to be opened to an open cross section ofapproximately 300 mm²; the flow-control valve 16 would also have to havea rapid reaction time.

A pressure measurement transducer is shown at 34; it detects the gaspressure in the blowing-air feed line 8 and provides informationdescribing the gas pressure for transmission to the control system 18.The gas pressure of the blowing gas introduced into the preform can thusbe monitored by the control system 18 and subjected to furtherprocessing for control purposes. For example, on the basis of theblowing gas pressure, the control of the valves 16, 24 and/or 28 can becoordinated.

FIG. 2 shows a schematic diagram of the fluid-mechanical configurationof the blowing-air feed of a blowing air supply 40 via a blowing airdistributor 42 to the blow mold 12 according to another embodiment. Incontrast to FIG. 1, a volumetric flow rate sensor 44 is provided in thepreblowing circuit, that is, in the area of the gas line 6 between theblowing-air feed line 8 and the distributor 42. The volumetric flow ratesensor is in particular arranged downstream from the flow-control valve16. The flow-control valve position sensor 20 in FIG. 1 is also missinghere.

The volumetric flow rate sensor 44 measures the volumetric flow rate ofthe blowing air through the gas line 6 and transmits this informationvia a signal converter 46 to the control system 18. The informationprovided by the volumetric flow rate sensor 44 is used for a control orautomatic control algorithm, which is processed by the control system18. The volumetric flow rate through the gas line 6 can be provided as acontrol variable. The volumetric flow rate sensor 44 can also bearranged upstream of the flow-control valve 16.

The detailed change over time in the blowing air volume introduced intothe preform can be derived from an evaluation of the informationprovided by the volumetric flow rate sensor 44. This offers theadvantage, for example, that, as a result, direct conclusions can bedrawn concerning the expansion behavior of the preform 10 during thepreblowing phase. It is conceivable, for example, that, as the profilestored in the control system 18 for controlling the flow-control valve16 is being implemented, differences may occur in the shaping of thepreforms 10. This can be attributable to the fact that, for example, thematerial of the preforms comprises deformations or other irregularities.Through the knowledge of the volumetric flow rate during the preblowingstep, upper and/or lower limit values can be determined, between whichthe blowing air volume introduced into the preform 10 during thepreblowing step must remain. Thus the production of defective containerscan be prevented. When a volumetric flow rate sensor 44 is used, it ispossible to draw conclusions as to whether or not the volume of blowingair during the preblowing step is within this corridor formed by thelimit values. Alternatively, the volumetric flow rate conducted throughthe flow-control valve 16 can be calculated by the control system 18 ifthe pressure difference between the inlet and the outlet of theflow-control valve 16 and the flow-control valve position are known.

In principle, it is not provided that the measurement variable of thevolumetric flow rate sensor 44 exerts a direct influence on the settingof the flow-control valve 16. The direct influence, however, can beprovided if the control system 18, on the basis of the measurementvalues of the volumetric flow rate sensor 44, recognizes that theblowing air volume Introduced into the preform 10 overshoots orundershoots the defined corridor limits. In addition, a directconnection of the measurement value of the volumetric flow rate sensor44 to the setting of the flow-control valve 16 can be provided by way ofa closed-loop control circuit.

The use of a volumetric flow rate sensor 44 also offers the advantagethat aging phenomena or functional incapacities of a flow-control valve16 or of a blow-molding station can be detected.

FIG. 3 shows a schematic diagram of the blowing-air feed of ablow-molding machine to the blow mold 12, in which a stepwise-adjustableflow-control valve 36 is arranged in the preblowing circuit, i.e., inthe area of the gas line 6 between the distributor 42 and theblowing-air feed line 8. In contrast to the exemplary embodiments ofFIGS. 1 and 2, in the present case the blowing-air feed to the preform10 inside the blow mold 12 is adjusted in a stepwise manner during thepreblowing step. This can be advantageous when a continuously variableand thus finely adjustable setting of a flow-control valve is notnecessary for the preblowing process. In principle, a flow-control valve36 which can move only in a stepwise manner is of simpler design than acontinuously variable flow-control valve 16 as present in FIGS. 1 and 2.The flow-control valve 36 is less expensive and mechanically lesscomplex. It is readily conceivable that the stepwise-adjustableflow-control valve 36 could also be combined with a flow-control valveposition sensor 20 of FIG. 1 and/or with a volumetric flow rate sensor44 of FIG. 2.

FIG. 4 shows a purely schematic diagram of the fluid-mechanicalconfiguration of a blowing-air feed from a blowing air supply 40 via adistributor 42 to a blow mold 12 of a blow-molding station known fromthe prior art. In contrast to the exemplary embodiments of FIGS. 1-3according to the invention, a manually adjustable flow-control valve 56for controlling the volumetric flow rate of the preblowing air isprovided in the preblowing circuit in the area of the gas line 6 betweenthe distributor 42 and the blowing-air feed line 8. Upstream from theflow-control valve 56, a blowing air valve 52 controlling the gas line 6is arranged, which can be actuated by the control system 18 by way ofthe signal converter 54. Further upstream from the valve 52, a tank 50is connected to the gas line 6; this tank is configured as anintermediate gas reservoir and makes it possible to distribute theblowing air to additional blowing stations of the blow-molding machine.Further upstream from the tank 50, a pressure control valve 48 isprovided, which is subjected on the inlet side to the primary blowingpressure from the distributor 42. The pressure control valve 48 reducesthe pressure of the blowing air from the distributor 42 to about 10bars. On the outlet side, the pressure control valve communicates withthe tank 50. As needed, the tank 50 distributes the blowing air toseveral blow-molding stations of a blow-molding machine.

The disadvantage of the way in which the blowing air is suppliedaccording to FIG. 4 is associated with the use of an expensive andmaintenance-intensive pressure controller 48 and of a tank 50, which isresponsible for distributing the preblowing air to the individualblow-molding stations. The preblowing circuit thus becomes complex andfault-prone. In addition, the pressure of the preblowing air can only beset centrally at the pressure control valve 48 jointly for all of theblow-molding stations of the blow-molding machine.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the inventive principles, it will beunderstood that the invention may be embodied otherwise withoutdeparting from such principles.

We claim:
 1. A control device for controlling a blow-molding machine forproducing a container of thermoplastic material, wherein a blowing gasfor blow-molding the container is introduced into a thermallyconditioned preform, while the preform is held in a blow mold of theblow-molding machine, wherein the control device comprises a controlunit set up to generate a control signal for setting a flow-controlvalve that determines a volumetric flow rate of the blowing gas, whereinthe control unit generates the control signal for setting theflow-control valve as a function of a position signal representing aposition of the flow-control valve.
 2. The control device according toclaim 1, wherein a nominal profile is stored in the control unit, whichnominal profile describes a change over time in the volumetric flow rateof the blowing gas, and the control unit generates the control signalfor setting the flow-control valve as a function of the nominal profile.3. The control device according to claim 1, wherein a desired upperand/or lower limit value for the volumetric flow rate of the blowing gasis stored in the control unit for at least one point in time of theblow-molding, and the control unit compares the volumetric flow ratesignal containing information on the flow quantity of the blowing gasintroduced into the preform with the limit values and determines adifference between the volumetric flow rate signal and the limit values.4. The blow-molding machine for producing a container of thermoplasticmaterial, in which a blowing gas for blow-molding the container isintroduced into a thermally conditioned preform, while the preform isheld in a blow mold of the blow-molding machine, wherein theblow-molding machine comprises a control device according to claim 1,wherein the blow-molding machine is controlled by introducing a blowinggas for blow-molding the container into a thermally conditioned preform,while the preform is held in a blow mold of the blow-molding machine,including introducing the blowing gas during a preblowing phase into thepreform so that the thermally-conditioned material of preform approacheswalls of the blow mold; introducing the blowing gas during a followingfinish-blowing phase into the preform so that the thermally-conditionedmaterial of the preform is pressed into contours of the walls of theblow mold; determining a volumetric flow rate of the blowing gas atleast during the preblowing phase by an adjustable flow-control valvecontrollable by a control unit that generates a control signal forsetting the flow-control valve; and adjusting the flow-control valve asa function of the control signal.